In an article by P. D. Hanstead entitled "A new ultrasonic focusing system for materials inspection" in the British periodical "Journal of Physics D: Applied Physics", Vol. 7, 1974, pp. 226 to 241, as well as in British patent No. 1,364,254, a direct-fault-display system by ultrasonic echography using the schlieren (strioscopic) method is described in which the object to be examined is coupled to an electroacoustic transducer subjecting it to periodic ultrasonic pulses and in which acoustic waves reflected by structural discontinuities (faults) are retransmitted by the transducer to an acousto-optical interaction cell that includes a transparent tank containing a piezo-optical liquid medium (water) and an acousto-optical system with two aligned cofocal acoustic lenses (afocal system). The acoustic waves which propagate in the medium fork streaks or schlieren at image points correlated with respective object points that are made visible by an optical system illuminated by a stroboscopic source.
However, such a system has large acoustic-energy losses due to attenuation through the object, transducer, piezo-optical medium and acoustic lenses and multiple reflections at interfaces between media of different densities which weaken the system's response.